The manufacture or fabrication of semiconductor devices often requires the careful synchronization and precisely measured delivery of as many as a dozen gases to a process chamber. Various recipes are used in the manufacturing process, and many discrete processing steps, where a semiconductor device is cleaned, polished, oxidized, masked, etched, doped, metalized, etc., can be required. The steps used, their particular sequence, and the materials involved all contribute to the making of particular devices.
As device sizes continue to shrink below 90 nm, the semiconductor roadmap suggests that atomic layer deposition, or ALD processes will be required for a variety of applications, such as the deposition of barriers for copper interconnects, the creation of tungsten nucleation layers, and the production of highly conducting dielectrics. In the ALD process, two or more precursor gases flow over a wafer surface in a process chamber maintained under vacuum. The two or more precursor gases flow in an alternating manner, or pulses, so that the gases can react with the sites or functional groups on the wafer surface. When all of the available sites are saturated from one of the precursor gases (e.g., gas A), the reaction stops and a purge gas is used to purge the excess precursor molecules from the process chamber. The process is repeated, as the next precursor gas (i.e., gas B) flows over the wafer surface. A cycle is defined as one pulse of precursor A, purge, one pulse of precursor B, and purge. This sequence is repeated until the final thickness is reached. These sequential, self-limiting surface reactions result in one monolayer of deposited film per cycle.
The pulses of precursor gases into the processing chamber is normally controlled using on/off-type valves which are simply opened for a predetermined period of time to deliver a desired amount of precursor gas into the processing chamber. The amount of material (mass) flowing into the process chamber is not actually measured.
What is still desired is a new and improved system and method for measuring and delivering pulsed mass flow of precursor gases into semiconductor processing chambers. Preferably, the system and method will actually measure the amount of material (mass) flowing into the process chamber, and provide reliable measurements. In addition, the system and method will preferably provide highly repeatable and precise quantities of gaseous mass for use in semiconductor manufacturing processes, such as atomic layer deposition (ALD) processes.
Furthermore, the system and method for measuring and delivering pulsed mass flow of precursor gases will preferably be designed to extend the life of valves used by the system and method. In addition, the system and method will preferably provide back-up or secondary valves for operation in the event a primary valve of the system should fail, so that the system and method are even more reliable and downtime of semiconductor processing chambers using the system and method is minimized.